Process for determining the stresses in structural elements and in completed constructions



Jan. 21, 1936. J. MATHAR PROCESS FOR DETERMINING THE STRESSES IN STRUCTURAL ELEMENTS AND IN COMPLETED CONSTRUCTIONS Filed Aug. 10, 1952 Patented Jan. 21, 1936 OFFICE PROCESS FOR DETERMINING THE STRESSES IN STRUCTURAL ELE- MENTS AND IN COMPLETED CON- STBUCTIONS Josef Mathar, Aachen, Germany, assignor of one-half to Schumag Schumacher Metallwerke Aktiengesellschaft fiir Prazisionsmechanik,

Aachen, Germany Application August 10, 1932, Serial No. 628,261 In Germany August 17, 1931 2 Claim.

The present invention relates to a process for the determination of the stresses in structural component parts and in completed constructions. the load conditions of which remain unaltered 5 during the stress-measurement.

Processes are already known for estimating additional stresses occurring in structural parts during the stress-measurement but which are not existing before such measurement. These processes can, however, be applied for such measurements only, that is to say, for additional stresses.

There has also been disclosed a process for the calculation of intrinsic stresses, meaning thereby stresses which have been set up in the testpieces prior to their subjection to stress by external forces. Provided a test piece, which may be either a circular or a flat bar or of any similar form, can undergo longitudinal stresses only,

' according to the process in question a layer was removed from the test piece, and the layer extending over a considerable length either over its entire circumference or over a width equal to that of a complete lateral surface. Thus the residual test piece undergoes either a contraction or adilatation along the length of the removed ,layer, this depending on whether compressive or distensive force was at work in the layer in ques-' tion. When a unilateral removal of a layer occurs, this gives rise also to a certain flexure in the residual part. The length-differenbe or the flexure or both is measured and this is made a basis for the calculation of the stress which was present in the removed layer. One drawback of this process is that it is not suitable for the determination of stresses other than "intrinsic" ones and that it is primarily inappropriate for the determination of stresses in completed structures seeing that these cannot have been so far weakened as to occasion any appreciable deformation in the rest of the structure and because also the production of such like damages is, as a rule,impossible in completed structures. A further inherent drawback is that the process is dependent on the shape of the test piece and on the stress distribution. Each particular form of body and every supposed type of stress requires a special treatment and special calculations. Suppose, for instance, that not merely longitudinal stresses but also transverse stresses are present in the test piece, the manner of measurement and also the calculation would require to be quite different. A final disadvantage is that the process yields accurate results only when the stress is uniformly distributed over a fairly large area.

According to the process of the invention which is suitable for determining intrinsic (body) stresses in the test piece and also for calculating the stresses in completed parts of building construction subjected to forces from without, all 1 the drawbacks referred to are removed.

According to this process, the test piece or structural element is damaged at a minute position only so as to yield the stress while all remaining fibres of the piece, also those situated on 15 the same surface as the damage, remain intact. The fibre-bundle which is cut through by the damage need be only very small in proportion to the total of undamaged fibreseven on the top surface. In order, for instance, to test even the heaviest bridge-girders it will suffice to weaken by a bore of, at the utmost, 6 mm. diameter and of a. depth somewhat similar. Therefore this damage is smaller than those produced by the holes for the rivets in the girders. In the case of so 25 diminutive a damage there occurs practically no alteration in the total length or any fiexure in the structural elements but merely a compression or extension in the extremities of the fibres cut through. so

The local disturbance in the fibre stress is, owing to the slight damage, appreciable within such narrow limits in the tested body that the fibres, even at some distance from the damage, remain practically unchanged in stress. As, in 35 addition, the cross-section is only minutely weakened, the form and size of the body to be tested may be neglected so far as the measurement is concerned. Moreover, it is only in the immediate neighbourhood of the edge of the minute 40 damage that purely local warping of the fibre extremities will occur and on this the form and size of the total cross-section is without effect. Hence for the calculation results it becomes a matter of indifference whether the mutual change in distance between the two fibre extremities facing each other at the damaged spot or whether the distance of only one of these from a fixed point situated at a given distance, is measured. In the 50 former case the change in distance is merely twice as great as in the second case.

From the results of the measurement the stress obtaining at the test position will be ascertained for instance with the aid of relations between measurements and stresses determined for similar material and for the same damage.

Instead of the damage taking the form of a bore it may also be carried out by an incision or some other suitable method.

From one and the same test or damage it is possible to determine stress conditionsinvolving not only one but several axes by employing measuring points for the determination of the fibreextremity strains, situated in different directions around the position to be damaged.

In order to determine the amount of fibre-extremity warping it will sufiice in many cases to measure simply the distance between the extremities before and after damage if finely-measuring instruments are employed for the purpose. Even then, however, the utmost care must be taken to avoid measurement-errors.

A more reliable result would be obtained if, firstly, a measuring instrument is mountedat the position to be tested and if then the body is damaged by boring a small hole at one or more supporting points situated as closely as possible. by which method every, even the most minute, change in the distance between the points fixed by the measuring instrument will be indicated without error because the measuring instrument remains in close contact with the tested body throughout the entire operation.

In order that the invention may be more clearly understood, reference is had to the accompanying drawing which illustrates diagrammatically a preferred mode of carrying out the improved process.

In the drawing:

Figure 1 is a view in elevation, partly in section, illustrating a preferred form of the extensometer.

Figure 2 is a diagrammatic view illustrating an example of a calibration curve obtained as a result of theapparatus.

a is the test piece, b the spot at which the stress should be determined, and c the extensometer.

The extensometer instrument 0 shown in the illustration possesses a rigid feeler tip d and a movable feeler tip Before applying the instrument to the body to be tested, the movable feeler tip is locked in the instrument by means of the screw g in such a manner that the pointer h, which is connected with the feeler point 1 through a leverage of large ratio of transmission, assumes the zero position. When the instrument has been clamped to the test body and two points have thus been fixed by the feeler tips cl and j, the movable feeler tip is unlocked and a slight bore made in the test body at b quite close to the feeler tip I. It is preferable to surround the drill z with a protective sleeve k for preventing the cuttings from coming into contact with the fine measuring instrument. If a tensile stress prevails at the testing spot b, the result of the boring operation will be that the fibers lying in the direction of force and out through by the drill contract at the margin of the bore. This will produce a displacement of the fixing point) toward the right, while the fixing point 01 reserves its initial position. This displacement of the fixing point is transmitted on an enlarged scale to the pointer h, causing the same to deflect more or less toward the right according to the size of the tensile stress prevailing at b. When a compressive stress exists at the spot of test 17. the boring at b entails a compression of the fibers and a displacement of the fixing point 1 toward the left, so that the pointer h is more or less deflected toward the left in accordance with the size of the compressive stress.

The instruments adapted for use in carrying out applicants process may be of varying constructions. They may, for instance, be made of a construction in which the rigid feeler tip is not set at d but likewise near the margin of the spot to beinjured, such as at d opposite the movable feeler tip I. The change in distance between the two feeler tips will then be doubled, because in addition to the fiber distortion at f, the fiber distortion at 11' will enter into eifect.

While preferring the particular arrangement above described, it is quite apparent that, as previously indicated, the stress may also be determined by measuring the distance between the measuring points by means of finely divided measuring instruments before and after the bore or other injury is made. The invention essentially consists in locating on the body to be tested at a predetermined spot at least two fixing or measuring points, making in the body by boring, cutting or the like near one or more of the measuring points, so minute an injury asto produce practically no change of the form of the test body as a whole but only a very limited local disturbance of the fiber stress with a resultant change of distance of the measuring points, the size of which yields the stress prevailing at the test spot.

The calibration of the extensometer can be easily obtained with simple means for a predetermined diameter of the drill in the following manner.

Test bars of rectangular cross-section are put under tensional or compressive stress. The extensometer is then applied to the test bar and the hole bored. In accordance with Figure 2, there can then be obtained a curve A which indicates the stress in dependence of the indication (deflection) of the extensometer. The said figure illustrates an example of a calibration curve obtained in this manner, this curve relating to a mild steel with a Youngs modulus of 2,000,000 kg/cm If many measurements with test pieces of one and the same material should be carried out, the extensometers themselves may be provided with a scale directly calibrated for stress (stress in kg/mm As regards the distance of the measuring point from the edge of the hole, a definite distance can easily and accurately be fixed and maintained with the aid of a gauge of known thickness, which is held between the drill and the extensometer when the same is being placed on the test body and withdrawn again when the instrument has been fixed on the body.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The process herein described for the determination of stremes in structural elements and completed constructional works which consists in marking two measuring points on the body under stress to be tested, destroying the equilibrium of the stresses present in the body by removing a minute part of the material at a spot in the immediate neighborhood of one of the measuring points, though without materially impairing the carrying capacity of the body, and measuring the alteration thus produced in the distance between the measuring points as an index of the stress prevalent at said spot in the body.

2. The process herein described for the determination of stresses in structural elements and completed constructional works which consists in applying an extensometer to the body to be tested, locating on the body two measuring points by means of the said extensometer, destroying with the extensometer constantly applied to the 10 body the equilibrium of the stresses present in the body by removing a minute part of the material at a spot in the immediate neighborhood or one of the measuring points, though without materially impairing the carrying capacity of the body, and determining from the alteration thus produced in the distance between the measuring points an index of the stress prevalent at the said spot in the body. 

